In a motor vehicle hydraulic brake system the necessary fluid, that is to say the brake fluid, is situated in the fluid reservoir, the reservoir filling level being monitored by the magnet, which serves for operating switching contacts. If the filling level of the brake fluid falls, the switching contacts are operated. This signal is evaluated in an electronic control unit and the driver of the motor vehicle can be alerted by a light or the like. At the same time it must be ensured that the fluid reservoir is always filled with a minimum quantity of brake fluid, so as not to put the working of the brake system at risk. Devices in which the float operates a reed contact as soon as the float assumes a predefined position (switching point), at which the reservoir filling level falls below a defined minimum, are generally used for monitoring a reservoir filling level. The switching of the reed contact triggers a warning signal perceptible to the driver, who should then take suitable remedial measures.
DE 37 16 135 A1, which is incorporated by reference, for example, discloses such a fluid reservoir. In order to exercise better control over the dynamics of the brake fluid, the inside walls, which define the chambers in the reservoir interior space, are usually equipped with fluid connections in the form of gaps, overflows etc.
If the fluid reservoirs are also equipped with a device for monitoring a reservoir filling level (warning device), the guide sleeve and the float are provided in a separate float chamber, on account of the fluid dynamics, which can give rise to the following problems: in braking, acceleration or cornering and under any combination of these demands, the shape of the reservoir may cause the fluid to flow out of the float chamber too rapidly, so that the float falls and operates the warning device. Operation of the warning device gives rise to an unwanted display and/or to entry in a vehicle fault memory, and the driver is required to go to a service workshop in order to have the fault cleared.
In order to prevent a rapid outflow of fluid from the separate chamber, the fluid connection may be provided in the form of a very small gap, for example, although this has a detrimental effect on the return of fluid into the chamber. This can likewise give rise to an incorrect operation of the warning device.
One remedy is afforded by the use of a timing element, which delays the operation of the warning device. However, the additional costs are seen as a disadvantage.